Your running economy (how much oxygen you use to run at a given speed) is one of the most important factors determining your racing performance. Just as some cars are more economical than others in their consumption of gasoline, some runners are more economical than others in their consumption of oxygen. A more economical runner uses less oxygen to maintain a given pace, thus burns less fuel and generates less heat, both of which are an advantage for distance runners. It is not unusual to find differences of over 20 percent in running economy between competitive runners.

Because the faster you run, the more oxygen you use, running economy should only be compared at a specific running pace. Some runners are highly economical at slow paces but become relatively less economical (in comparison to other runners) at faster speeds. Since the ability to consume oxygen is limited, being more economical is a huge performance advantage.

Despite 30 years of research, it is still not entirely clear why some runners are more economical than others. As with most human characteristics, genetic differences determine the range of your running economy. You can, however, improve your running economy through training, determining where you fall within that range.

Various studies have found that several factors influence running economy. Let’s take a look at each:

Muscle fiber type: The proportion of slow-twitch versus fast-twitch fibers in your muscles partially determines your running economy. Slow-twitch muscle fibers use oxygen more efficiently, and the most economical runners tend to have a higher proportion of slow-twitch fibers than their less economical (and generally slower) associates. Unfortunately, you cannot convert your fast-twitch fibers to slow-twitch fibers, but with endurance training fast-twitch muscle fibers gain more of the characteristics of the more economical slow-twitch fibers.

Energy return: During the landing phase of your stride, your muscles lengthen and store energy, which is then released as you push off. The ability of the muscles to store and return energy in a spring-like action varies between runners and may explain differences in running economy, especially for races at the shorter end of the distance running spectrum with faster speeds, higher impact forces, and greater energy storage and return factors.

Vertical movement: The running stride includes a flight phase when both feet are off the ground, so a degree of vertical movement is inevitable. Because the objective of running is to move forward, however, excessive up-and-down movement wastes energy. One adaptation to high-mileage training may be to develop a more economical stride by reducing vertical motion. Given enough time, and a few thousand miles, you may develop from a relatively bouncy 5,000m runner into a more economical marathoner.

Stride length: Running speed is determined by stride length times stride frequency. Every runner has an optimal stride length for each running speed, based on a complex combination of leg length and muscular characteristics. If you overstride or run with a shorter than optimal stride length you will use more oxygen. Fortunately, the vast majority of runners inherently select the optimal stride length for each running speed. It is not known exactly how we do it, but presumably running with the most economical stride length leads to less perceived exertion, and we naturally seek to run as comfortably as possible. Over time, it is possible to increase your optimal stride length at a given pace, which may lead to further improvements in economy.

Other aspects of running technique: A common misconception is that coaches can look at a runner and tell whether she is efficient or economical. Most of the differences in running economy, however, occur inside the muscles and are not obvious to even the most experienced coach. Actively correcting seemingly sloppy technique—such as arms crossing the body—has not been found to improve running economy, likely because the apparently sloppy technique is actually compensating for other variables. Over the course of months and years however, subtle changes in running technique do seem to lead to subtle improvements in running economy as your running "skill" improves. This may be due to improved ability to recruit the most efficient muscles’ fibers for the task at hand. With increased experience, you may also improve your ability to relax opposing muscles and reduce the use of stabilizing muscles, reducing your consumption of oxygen.

Flexibility: Interestingly, runners who are more economical tend to be relatively inflexible. Reduced flexibility may increase the ability of the muscles to store and return energy, or increase leg stability (so stabilizing muscles are used less). Before you quit stretching, however, keep in mind that the relationship between flexibility and running economy has only been shown between runners, and does not prove that a given individual will improve her running economy by becoming less flexible.

Fatigue level: As you get tired, you use more oxygen to run at a given pace. This may be due to reduced energy return and increased use of additional muscles’ fibers as the muscles fatigue, among other factors. One of the major benefits of tapering training before a race is an improvement in running economy.